1. Field of the Invention
The present invention relates to an electronic attenuation value control circuit in which semiconductor devices are substituted for mechanical components, and more particularly to an electronic volume control circuit used in a television receiver, a radio receiver, an audio set, etc.
2. Description of the Prior Art
An electronic attenuation value control circuit using semiconductor devices has been employed in place of a mechanical attenuation value control circuit constituted by a variable resistor or a mechanical switch. The electronic attenuation value control circuit has a string of resistors and a plurality of electronic switches which are fabricated in a semiconductor integrated circuit device. The respective resistors are made of, for example, a polycrystalline semiconductor layer formed on a semiconductor substrate, and each of electronic switches is composed of transistors formed in the substrate. The string of resistors is connected between a signal input terminal and a reference potential point (a ground point, for example). The signal input terminal is supplied with an analog signal such as an audio signal. Accordingly, the attenuated signals of the analog signal appear at the respective interconnection point of resistors in accordance with the resistance ratio of the respective resistors. Each of the interconnection points of the resistors is connected to one end of the respective electronic switches. The other ends of the switches are connected in common to a signal output terminal. As a result, by making either one of the electronic switches conductive, the attenuated signal is derived from the signal output terminal, and the volume from a loudspeaker is changed.
It will be easily understood that the electronic attenuation value control circuit employed in a hi-fi audio set is required to produce the attenuated signal with a low distortion and to realize a sufficiently low switch-on resistance and a very high switch-off resistance of the respective electronic switches. For these purposes, each of the electronic switches has first and second transistors connected in parallel. These transistors are IGFET's (Insulated Gate Field Effect Transistor) such as MOS (Metal-Oxide-Semiconductor) transistors having different conductivity types from each other. The first transistor has a channel region which is a surface portion of a semiconductor substrate of one conductivity type, whereas the second transistor has another channel region which is formed at the surface portion of the substrate as so-called "well region" of the opposite conductivity type to that of the substrate. Bias voltages are usually applied to the substrate and the well region to reverse-bias the p-n junction of the source and drain regions of the respective transistors. The first and second transistors are made conductive by supplying voltages having different polarities and being higher than threshold voltages of the respective transistors to their gates, so that the electronic switch takes an on-state. However, since the analog signal is transmitted through the switch, the difference occurs between the potential at the source or drain and the potential at the channel region in the respective transistors. Further, the voltage difference is changed. For this reason, the so-called "back-bias effect" occurs to change the threshold voltage of the transistor. Thus, the threshold voltage is varied in response to the potential at the channel region, and so this region is called as "back-gate". In contrast to the back-gate, the gate electrode of the transistor is called "front-gate". When the threshold voltage is changed by the back-bias effect, the transconductance of the transistor is varied. As a result, the distortion of the analog signal is deteriorated. A control circuit is also formed in the semiconductor substrate for selecting either one of the electronic switches, and some transistors in the control circuit has other surface portions of the substrate as their channel forming region. Therefore, it is impossible that the bias voltage to the substrate is changed. On the other hand, the well region in which the second transistor is formed can be formed separately from other well regions. In other words, the back-gate of the second transistor can be connected to one end or the other end of the switch. Consequently, the potential at the back-gate of the second transistor is made equal to that at the source or drain of the same, and the back-bias effect is suppressed to reduce the deterioration in the distortion characteristic of the analog signal. If the back-gate of the second transistor, i.e., the well region is maintained to be connected to one or the other end of the switch, it is also subjected in a switch-off state to the potential change in accordance with the level variation of the analog signal. For this reason, the second transistor cannot be made nonconducting perfectly. In order to eliminate this shortcoming, the back-gate of the second transistor is connected in the switch-off state to a potential point that deeply reverse-biases the p-n junctions of the source and drain of the second transistor.
As described above, each of the electronic switches is composed of the first and second transistors of the different conductivity types from each other, and further has a back-gate control circuit which switches over the potential applied to the back-gate of the second transistor in the switch-on state and the switch-off state. Accordingly, such an electronic switch has a sufficiently low switch-on resistance and a very high switch-off resistance, and suppresses the deterioration in distortion of the analog signal transmitted therethrough. The electronic volume control circuit using the above-mentioned switches is applicable to the hi-fi audio set.
However, the change in the voltage at the back-gate of the second transistor is differentiated by the p-n junction capacitance or the stray capacitance in the transistor. Consequently, a noise voltage in a pulse shape appears at the other end of the switch. The other ends of the electronic switches are connected in common to the signal output terminal. Further, the conductive states of two parallel-connected electronic switches are switched over when the attenuation value is changed. For this reason, a noise signal having a relatively large level is produced at the signal output terminal every time when the attenuation value is changed. Since the state of the electronic switch is changed over instantaneously, the frequency of the noise signal is considerably higher than the audio frequency. Accordingly, the change in level at the signal output terminal caused by the noise signal may be mitigated by using a low-pass filter, for example. However, the voltage level of the noise signal is considerably large, the signal output terminal takes a relatively high potential change. The potential change at the output terminal is further amplified by a power amplifier of the next stage, so that a pop noise is generated by a loudspeaker. The speaker or other signal processing circuits may be destroyed by the noise signal.